System for producing two simultaneous records of high energy electrons in an electron microscope



May 5, 1970 L. L. MARTON 3,510,649 SYSTEM FOR PRODUCING TWO SIMULTANEOUS RECORDS OF HIGH ENERGY ELECTRONS IN AN ELECTRON MICROSCOPE Filed Oct. 5, 1967 i;12 HIGH ENERGY ELECTROMS 7O VIEWING SCREEN 14 TRANSM SSIVE DYNODE 16 WI-24 PHOTOGRAPH/C EMULSION -20 ACCELERATING ELECTRODE L- {7 12 HIGH ENERGY ELECTEOMS Elli j:::;::1 ::1 :::12i::3 C::::. 10 VIEWING SCREEN K20 ACCELERATING ELECTRODE J 25 24 PHOTOGRAPHIC EMULSION 52 3O FRONT SURFACE DVNODE W HIGH ENERGY ELECTRONS 14 TRAMSMISSIVE DYNODE 24 PHOTOGRAPHIC EMULSION 20 AC ELEQATING ELECTRODE J25 *24- PHOTOGRA PHIC. EMULSION J0 FRONT SURFACE DVNODE INVENT OR L. L. Morton AGENT United States Patent O SYSTEM FOR PRODUCING TWO SIMULTANEOUS RECORDS OF HIGH ENERGY ELECTRONS IN AN ELECTRON MICROSCOPE Ladislaus L. Marton, Washington, D.C., assignor to the United States of America as represented by the Secretary of Commerce Filed Oct. 3, 1967, Ser. No. 672,648 Int. Cl. G01j 37/22; G01n 23/04 US. Cl. 250-495 1 Claim ABSTRACT OF THE DISCLOSURE Electrons having energies above 100-300 kev. are inefiiciently recorded by photographic emulsions. A dynode (transmissive and/r front surface) is placed in the path of such electrons to produce secondary electrons; these secondaries are suitably accelerated, to efficiently expose the emulsion.

BACKGROUND OF THE INVENTION This invention relates to high voltage electron microscopes, and more particularly to dynode arrangements for recording the high energy electrons projected by such microscopes.

Electron microscopes typically have a maximum accelerating voltage of 100 kv.; some, however, have maximum accelerating voltages of 300 kv., 500 kv., and even in excess of 1 mv. As is well known, it is difiicult to obtain a photographic record of the electrons accelerated by these voltages. At energies above 300 kev., especially, most of the electrons pass through the photographic emulsion without sufiicient useful collisions with the silver halide to form a latent image.

SUMMARY OF THE INVENTION The present invention records high energy electrons by providing a secondary electron emitting electrode or dynode in the path of the electrons. The high energy electrons impinging on the dynode produce secondary electrons, which are then suitably accelerated to an adjacent photographic emulsion. In this manner, the emulsion is exposed to both the primaries and the secondaries, and thus records an intensified electron image corresponding to the original high energy electron image.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of a transmissive dynode embodiment of the invention;

FIG. 2 is a schematic diagram of a front surface dynode arrangement; and

FIG. 3 is a schematic diagram of a combined transmissive and front surface dynode arrangement according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An electron miscroscope comprises an evacuated chamber having a heated cathode for emitting electrons, an anode for accelerating the electrons, and a condensing lens for focusing the electrons onto a thin specimen to be examined. The specimen scatters the electrons according to the density at each of its points, and hence the emerging electrons form an electron image of the specimen. The image is magnified by an objective lens (in many instruments also by an intermediate lens) and the magnified image is projected by a projector lens onto a fluorescent viewing screen. To record the electron image, the screen may be used as a shutter for the photographic emulsion. In this case the screen is rotated or 3,510,649 Patented May 5, 1970 otherwise moved out of the path of the electrons, to permit the electrons to fall on and expose a photographic emulsion positioned beyond (usually, beneath) the viewing screen. The present invention provides various dynode arrangements between the viewing screen and the photographic emulsion, whereby the emulsion can efiiciently record highly accelerated electrons, especially those accelerated to about 300 kev. and more.

In FIG. 1, the dashed rectangle 10 schematically represents the fluorescent viewing screen (and shutter, if a separate shutter is used) of a high voltage electron microscope, such as described above. The viewing screen 10 is normally positioned in the path of the high energy electrons 12 projected by the microscope, and can be moved, by the operator, out of the path of the electrons 12, to permit the electrons to impinge on a planar transmissive dynode 14 that is disposed perpendicular to the microscope axis, just beneath the viewing screen 10. The transmissive dynode 14 is conventional, and comprises a thin, suitably supported metallic layer 15, and a layer of secondary electron emissive material 16, such as potassium chloride, magnesium oxide, or the like. The high energy electrons 12 pass through the thin metallic layer 15 and release several secondary electrons 18 from the secondary emissive material 16.

A planar metallic accelerating electrode 20 is disposed parallel to the transmissive dynode 14, opposite the sec ondary emissive layer 16 of the dynode; and a source of accelerating potential 22 is connected between the electrode 20 and the metallic layer 15 of dynode 14, to establish an electric field that accelerates the secondary electrons 18 to the electrode 20. A photographic emulsion 24 carried on a substrate 25 (or without a substrate) is interposed between the dynode 14 and electrode 20 to receive the accelerated secondary electrons. The accelerating potential is 5-10 kv. to impart sufiicient velocity to the secondary electrons 18 to efiiciently expose the photographic emulsion 24. Thus, the emulsion 24 receives, in addition to the primaries, several secondary electrons 18 of moderate energy. The moderate energies of the secondary electrons permit them to expose the emulsion in a quick, eflicient manner, while their plural numbers provide an intensified version of the original electron image. By making the spacing between the elements 16 and 24 the minimum compatible with safe operation of the high voltage, the resolution of the final image can be made comparable with that of direct recording, as the spreading of the electrons from each image point can be kept to a minimum.

As shown in FIG. 1, the high energy electrons 12 also pass through the emulsion 24, although they cause little exposure of the emulsion. The electrons may continue through the substrate 25, and possibly through the metallic electrode 20, depending on the thickness of the electrode 20 and the energies of the electrons 12.

FIG. 2 depicts a front surface dynode arrangement of the invention. A conventional planar front surface dynode 30, comprising a metallic support layer 31 and a secondary emissive layer 32, is disposed beneath the movable viewing screen 10, with the secondary emissive layer 32 facing the high energy electrons 12. The secondary electrons 34 produced by the impinging primary electrons 12 are accelerated, by the accelerating voltage 22, to the thin, metallic accelerating electrode 20, which is disposed above and parallel to the dynode 30. The accelerated secondaries 34 expose the photographic emulsion 24 on substrate 25, the substrate being held up to the electrode 20 by any suitable means (not shown).

In operation, the primary electrons 12 pass through the thin accelerating electrode 20, the substrate 25, the emulsion 24 (producing little if any exposure of the emulsion),

the secondary emissive layer 32, and the supporting layer 31 (depending on the thickness of the layer and the energies of the electrons). The secondary electrons 34 are emitted from the layer 32, and are accelerated by the 5-10 kv. voltage source 22 to the emulsion 24. Again, the plural number and moderate energies of the secondaries 34 produce an intensified image in the emulsion, Within a reasonable exposure time.

The embodiment of the invention shown in FIG. 3 essentially combines the embodiments of FIGS. 1 and 2. Thus, a transmissive dynode 14 and a front surface dynode 30 are disposed beneath the viewing screen 10, with the secondary emissive surfaces 16 and 32 thereof facing a common thin metallic accelerating electrode 20. The accelerating potential 22 has its positive terminal connected to the accelerating electrode 20, and its negative terminal connected to the metallic layers of the dynodes 14, 30. Two photographic emulsions 24 on substrate 25 are disposed on opposite sides of the accelerating electrode 20.

In operation, the primary electrons 12 pass through the entire assembly, producing the secondary electrons 18 and 34; the latter are suitably accelerated to the electrode 20, and expose the two emulsions 24, in an efficient, intensified manner. Thus the combined transmissive and front surface dynodes of FIG. 3 provide two simultaneous and identical records of the high energy electron image projected by the microscope. Multiplication by the usual means of combining several secondary emitting electrodes is possible, but reduces the definition of the image through spreading of each image point.

Although the invention has been described with reference to specific examples, it is to be understood that the invention is not limited to the details thereof, but includes all such variations and modifications as fall within the scope of the appended claim. By Way of example, it will be obvious to those skilled in the art that the parallel dynodes and accelerating electrodes of this invention can be suitably packaged, i.e., spaced and insulated and provided with electrical contacts, so as to form convenient assemblies for ready insertion into the photographic chamber of a high voltage electron microscope.

I claim:

1. In an electron microscope having means for projecting primary electrons of energies greater than about 300 kev. along a vertical path, and a viewing screen selectively disposed across said path for viewing said primary electrons or for permitting said primary electrons to continue along said path to be recorded, the improvement of means for rapidly and efficiently making two simultaneous records of said primary electrons, comprising:

first, second, and third parallel continuous planar elec trodes, each permeable to said primary electrons, and disposed in that order beneath said viewing screen, across said path,

first and second layers of secondary electron emissive material disposed on the lower surface of said first electrode and the upper surface of said third electrode, respectively,

first and second photographic emulsions disposed on the upper and lower surfaces of said second electrode, and

an accelerating potential source of about 5-1O kv. having its negative terminal connected to both said first and third electrodes and its positive terminal connected to said second electrode, to accelerate the secondary electrons ejected from said first and second layers of secondary electron emissive material by said primary electrons to said first and second photographic emulsions, respectively, thereby forming said two simultaneous records.

References Cited UNITED STATES PATENTS 3/1945 Baker 250-495 FOREIGN PATENTS 12/1965 Great Britain.

WILLMM F. LINDQU-IST, Primary Examiner US. Cl. X.R. 25 0-65 

